Aromatic polycarboxylic acids, such as terephthalic acid, are important chemical intermediates used for the production of industrially significant products, including polyester polymers, which can be used for fiber production and in the manufacture of containers, bottles and other molded articles.
Purified terephthalic acid (PTA) can be produced in a two stage process. Current technology for the manufacture of terephthalic acid involves the liquid phase oxidation of an aromatic feedstock, such as paraxylene, using molecular oxygen in a solvent. The oxidation solvent comprises a lower (e.g. C2-C6) aliphatic carboxylic acid, usually acetic acid and water, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine. Acetic acid is particularly useful as the solvent since it is relatively resistant to oxidation and increases the activity of the catalytic pathway for the oxidation of aromatic feedstock and reaction intermediates. The reaction is carried out in one or more stirred vessels under elevated temperature and pressure, in the range of about 150 to 250° C. and 6 to 30 barA respectively and typically produces crude terephthalic acid (CTA) in high yield, e.g. at least 95%. Under these conditions the CTA precipitates from the solvent in the oxidation reactor to form a slurry of CTA solids in oxidation solvent, which is maintained in suspension by agitation in the reaction vessels. The temperature of the slurry is reduced by passing through a series of crystallizers, each at successively lower pressure, before the CTA solids are separated from the oxidation reaction solvent to give the oxidation mother liquor. The separation of the CTA solids from the oxidation mother liquor occurs at positive pressure or under vacuum.
Typically, the solvent for the liquid phase oxidation is aqueous acetic acid and contains water resulting from the oxidation of paraxylene and other reaction precursors. The oxidation reaction is exothermic and generates aromatic carboxylic acid, reaction intermediates from the partial oxidation of the aromatic feedstock and by-products, comprising colour-forming compounds, volatile components, such as methanol, methyl acetate and methyl bromide and degradation products such as carbon dioxide, carbon monoxide (carbon oxides) and benzoic acid (BA).
The second stage of the production process is the purification of CTA by catalytic hydrogenation in aqueous solution. Typically, CTA solids are dissolved in water at high pressure (70-90 barA) and high temperature (275-290° C.), and hydrogenated over a fixed bed catalyst of palladium supported on carbon. The resulting solution is cooled as it passes through a series of crystallizers, where the purified terephthalic acid (PTA) is crystallized. The resulting slurry at a temperature in the range of about 140-160° C. is fed to a suitable continuous solid liquid separation device(s), such as a centrifuge or rotary filter, where the PTA solids are separated from the purification mother liquor stream, washed and then dried.
The oxidation reaction is maintained at a constant temperature by evaporation of the oxidation solvent which exits the reactor and returning condensed solvent, which can also be further cooled, to the reactor. In this way, the latent heat of the oxidation solvent is used to cool the oxidation reaction mixture. The vapor phase leaving the reactor, as vent gas, typically comprises vaporized acetic acid, water vapour and volatile reaction by-products, as well as non-condensable components including residual oxygen not consumed in the oxidation reaction, nitrogen (when air is used as a source of the molecular oxygen for the oxidation reaction) and carbon oxides.
Typically, water in the oxidation solvent in the oxidation reactor is maintained at a constant level by condensing the off-gas from the oxidation reactor to form a condensate, separating the condensate from the remaining gas stream and separating at least a portion of the water from the rest of the liquid condensate, before returning the remaining liquid condensate to the reactor as oxidation solvent. The excess water separated from the condensate can be fed to an effluent treatment unit for disposal.
The separation of water from the oxidation reactor off-gas condensate can most easily be carried out by distillation, with the lower aliphatic monocarboxylic acid-rich stream as the bottoms product and a water-rich stream as the tops product. A previous improvement to the production process was to eliminate the initial condensation step and consisted of feeding the oxidation reactor off-gas directly to a rectifier column. This column can be conveniently located above the oxidation reactor for the lower aliphatic monocarboxylic acid-rich stream to return directly to the oxidation reactor, although other configurations can also be used.
The oxidation reactor operates at elevated pressure and temperature and the vent gas from the oxidation reactor can be used to recover energy down stream of the rectification column. Energy recovery can be either direct or indirect; by heat exchange, for example to raise steam for use elsewhere in the process or by reducing the pressure of the gas stream through a machine, such as an expander. The expander can be used to recover energy, e.g. to power the air compressor feeding air to the oxidation process or to generate electricity.
To purify terephthalic acid suitable for the manufacture of polyester polymer to manufacture fibers, bottles, containers and other molded products, crude terephthalic acid is dissolved in water at high temperature and pressure before being hydrogenated over a heterogeneous catalyst. The purification stage can be used to remove reaction intermediates and by-products that are known to cause or correlate with color-formation in polyester polymer. In particular, the reaction intermediate p-toluic acid (p-Tol), an aromatic monocarboxylic acid, is additionally formed by the hydrogenation of the oxidation reaction intermediate 4-carboxybenzaldehyde (4CBA), a contaminant in CTA that has to be reduced or eliminated to produce PTA. As p-Tol is substantially water-soluble under the conditions used for purification, it is largely retained in solution as PTA solid crystals in multiple vessels downstream of the hydrogenation reactor and in the purification mother liquor, following separation of the PTA solids from the crystallized PTA slurry. Some p-Tol co-crystallizes with PTA, the amount being dependent on the process conditions. The p-Tol in solution is a yield loss for the conversion of paraxylene to PTA and restricts the use of the purification mother liquor as a source of water for use elsewhere in the production process.
To separate the lower aliphatic carboxylic acid from water recovered as condensate from the oxidation reactor off-gas, an adequate number of separation stages in the rectification column and sufficient aqueous reflux to the top of the column are required. However, the total flow of aqueous reflux back to the top of the column is constrained by maintaining the oxidation reactor water concentration at a target value. Typically, the aqueous reflux comprises a portion of the overheads product after condensing the water-rich vapor stream leaving the top of the rectifier column. The rest of the rectification column condensate, typically mostly water of reaction, is then removed from the rectifier overheads system.
An existing and alternative process is to remove a substantial portion of the overheads water condensate for use as make-up water on the purification stage of the PTA manufacturing process, with reflux to the rectifier being provided by a combination of pure plant mother liquor and a smaller portion of rectifier overheads condensate. The pure plant mother liquor contains significant concentrations of p-Tol, BA and other reaction intermediates and by-products, so this stream is returned to the rectifier several stages below the top of the column. The cleaner rectifier overheads condensate is returned to the top of the rectifier and removes volatile impurities from the pure plant mother liquor reflux from the rectifier overheads vapor stream, allowing the subsequent overheads condensate to be re-used elsewhere in the process. However, if the rectifier overheads condensate is used to supply the majority of the process water needs and if most or all of the pure plant mother liquor is returned as reflux to the rectifier, the amount of water reflux provided by rectifier overheads condensate is too low to remove the volatile components from the pure plant mother liquor and maintain the required purity of the rectifier overheads condensate without a disproportionate number of stages being required in the top scrubbing section of the rectifier. In particular, the concentrations of p-Tol and BA in the rectifier overheads condensate would make it unsuitable for use as make-up water on the purification stage of the PTA manufacturing process, where water substantially free of these components is required.
The consequence of these combined problems reduces the economic benefit of using a rectification column to separate water from the oxidation solvent, which would otherwise enable water recycle between the oxidation and purification stages of the PTA manufacturing process and simplify recovery of oxidation reaction intermediates from the purification process.